Triplet to singlet transition induced low efficiency roll-off in green phosphorescent organic light-emitting diodes

Phosphorescent organic light-emitting diodes (PHOLEDs) with an emitting layer of 4,4'-N,N'-dicarbazole-biphenyl codoped with phosphor fac-tri(phenylpyridine)iridium(III) [Ir(ppy)(3)] and fluorophore N,N'-dimethy-quinacridone (DMQA) are investigated. Predominant emission from DMQA due to the efficient energy transfer from Ir(ppy)(3) to DMQA is observed. Such an energy transfer results in the transition of Ir(ppy)(3) triplet to DMQA singlet, which reduces the Ir(ppy)(3) exciton lifetime and hence suppresses the triplet-triplet annihilation and triplet-polaron annihilation of Ir(ppy)(3) excitons, leading to dramatical reduction of the efficiency roll-off of the PHOLEDs. This transition of triplet to singlet strategy provides a method to improve the efficiency roll-off of the PHOLEDs. (C) 2010 Elsevier B.V. All rights reserved.